Human NF-kappaB (NF-KB) transcription factors regulate the expression of large number of genes involved[unreadable] in diverse biological activities. A sub-class of IKB proteins, known as prototypical IKB (IKBalpha, IKBbeta and[unreadable] iKBepsilon), inhibits NF-KB transcriptional activity by forming stable complexes with NF-KB dimers. These[unreadable] inhibited complexes define a paradigm of cell signaling where signal induced NF-KB activation is regulated[unreadable] by the degradation of IKB. The prototypical IKB proteins undergo rapid turnover in resting cell when present[unreadable] in the free state, compared to when present as complexes with NF-KB. Recent observations indicate that the[unreadable] prototypical IKBs are partly unfolded, which may stabilize upon binding NF-KB. Interestingly, critical[unreadable] segments of NF-KB that are involved in binding IKB are also unfolded in their free states. We hypothesize[unreadable] that stability of IKB proteins regulates their turn over, NF-KB recognition and the lifetime of IKB/NF-KB[unreadable] complexes in quiescent cells. A separate class of IKB proteins, represented by Bcl3 and IKBzeta, carry out[unreadable] different functions. It is likely that functional differences between these two classes of 1KB proteins may[unreadable] correlate with their differential stability. The focus of this proposal is to understand the mechanism of[unreadable] proteasome-mediated degradation of IKB and how their stability relates to their degradation. We have[unreadable] observed structural differences between prototypical IKB proteins such as the presence of a disordered insert[unreadable] in IKBbeta. We will test if IKBalpha and IKBbeta display differential specificity towards specific NF-KB dimers, and[unreadable] how structural differences between IKBalpha and IKBbeta play a role in such specificity-defining mechanisms.[unreadable] Finally, we will elucidate new structures of IKB/NF-KB complexes, which may provide insights into their[unreadable] biological functions. We will use X-ray crystallography, biochemistry and cell-based experiments to test our[unreadable] hypotheses.